1. Field of the Invention
This invention relates to telephone intercom systems, and more particularly to a system for allowing several self-contained station controller circuits adapted for use with relatively few links to be operated together to expand the number of links in the intercom system.
2. Description of the Prior Art
Intercom systems have long been used for selectively connecting a calling telephone station to a called telephone station to allow two-way communication. Generally, these systems connect the stations to each other through individual audio links, the number of which is far less than the number of stations in the system. Consequently, link sharing by the stations is necessary. A call is initiated when a calling station sequentially scans the audio links in search of an unused link and, when it finds an available link, connects itself to that link. The calling station then generates the address of the called station by dialing with either a dualtone multi-frequency device or a rotary dialing device. Link controllers associated with individual audio links decode the address of the called station and cause the called station to connect itself to the audio link to which the calling station is connected.
Recently, a multi-link intercom system of this type has been available from Tone Commander Systems, Inc., of Redmond, Washington, under the model designation ML8000 This system employs four audio links, and all of the station controllers and link controllers for the system operate on a four-link access period basis. The ML8000 system features a self-contained large-scale integrated station controller which is only capable of connecting its station to one of four audio links and which, like the remainder of the system, inherently operates on a four-link access period basis.
Although the number of audio links required in an intercom system for a given number of stations varies depending upon the extent to which each station utilizes the intercom, it is generally necessary to increase the number of audio links as the number of stations accessing those links increases. Thus, the need has developed for intercom systems having eight or more audio links. Expansion of four-link intercom systems employing self-contained station controllers to allow a greater number of links to be accessed requires that the self-contained controller circuits properly interface with each other. The interfacing requirement could undoubtedly be met by merely increasing the number of called station address lines, control lines and other circuitry in proportion to the increase of audio links. However, this solution would result in an unacceptable proliferation of the interconnection wiring in the system.
Intercom systems having a substantially larger number of audio links are also more difficult to test than systems having fewer links. Thus, it is desirable to simplify testing by allowing the system to selectively make some of the links busy to facilitate testing of other links.